Inkjet printing is typically done by either drop-on-demand or continuous inkjet printing. In drop-on-demand inkjet printing ink drops are ejected onto a recording medium using a drop ejector including a pressurization actuator (thermal or piezoelectric, for example). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the printhead and the recording medium and strikes the recording medium. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image.
Motion of the recording medium relative to the printhead during drop ejection can consist of keeping the printhead stationary and advancing the recording medium past the printhead while the drops are ejected, or alternatively keeping the recording medium stationary and moving the printhead. This former architecture is appropriate if the drop ejector array on the printhead can address the entire region of interest across the width of the recording medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead drop ejector array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a medium advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the medium advance direction as the drops are ejected from the nozzles. After the carriage-mounted printhead has printed a swath of the image while traversing the print medium, the recording medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
A drop ejector in a conventional drop-on-demand thermal inkjet printhead includes a pressure chamber having an ink inlet for providing ink to the pressure chamber, and a nozzle for jetting drops out of the chamber. Partition walls are formed on a substrate and define pressure chambers. A nozzle plate is formed on the partition walls and includes nozzles, each nozzle being disposed over a corresponding pressure chamber. Ink enters pressure chambers by first going through an opening in the substrate, or around an edge of the substrate. A heating element, which functions as the actuator, is formed on the surface of the substrate within each pressure chamber. The heating element is configured to selectively pressurize the pressure chamber by rapid boiling of a portion of the ink in order to eject drops of ink through the nozzle when an energizing pulse of appropriate amplitude and duration is provided.
Because portions of the ink itself are vaporized in a conventional thermal inkjet printhead, the composition and properties of the ink need to be compatible with rapid boiling without causing damage to the ink or the heating element. Such heating of some inks can cause degradation of ink components and ink properties. In addition, some inks can cause damage to the heating element or can cause a build-up of ink residue on the heating elements that can adversely affect the energy transfer efficiency of heat from the heating element into the ink. Furthermore, some inks that have desirable image forming properties do not have desirable bubble ejection properties, such as bubble nucleation factors, vapor bubble temperature, bubble formation speed and amount of force exerted on the heating element due to bubble collapse. Non-aqueous inks in particular can have poor performance in conventional thermal inkjet drop ejectors.
Because conventional thermal inkjet drop ejectors are incompatible with or have poor performance with certain types of ink, a common approach is to use piezoelectric inkjet printheads for such types of ink. However, in order to provide the required drop ejection force, piezoelectric drop ejectors require a much greater area on the substrate than thermal inkjet drop ejectors. As a result of the comparatively low packing density of piezoelectric drop ejectors, it is more difficult and more expensive to provide piezoelectric inkjet printheads having a high printing resolution and a small footprint.
Several patents, including U.S. Pat. Nos. 4,480,259, 6,312,109, 6,705,716 and 8,727,501, disclose a modified form of thermal inkjet where a bubble-driven flexible membrane is used to isolate the ink to be ejected from a working fluid that is used to provide the ejection force. FIG. 1 is adapted from FIG. 3 of U.S. Pat. No. 6,312,109 and illustrates a bubble-driven-membrane-type thermal inkjet drop ejector. In this example the drop ejector includes a dielectric substrate 21; a heating layer 22 overlaying the dielectric substrate 21, the heating layer 22 containing a resistor 23 for converting electricity into thermal energy; a heat dissipating layer 24 formed on the heating layer 22; a working fluid chamber 25 formed in the heat dissipating layer 24 and over the top surface of the resistor 23 for containing ink; a nozzle plate 26 formed over the heat dissipation layer 24 and having a nozzle 27; an ink chamber 28 formed in the nozzle plate 26 for containing ink; and a flexible membrane 29 formed between the heat dissipating layer 24 and the nozzle plate 26 to separate the working fluid chamber 25 from the ink chamber 28. Each ink chamber is formed with an ink channel 31 that receives ink from an ink supply (not shown). When a voltage pulse is applied to the resistor 23, a sudden outburst of thermal energy causes the working fluid to vaporize locally within a few microseconds, creating a bubble in the working fluid chamber 25. The expansion of the bubble causes the pressure within the working fluid chamber 25 to increase, and thus pushes the flexible membrane 29 outwards in the direction of added upward arrow 32. The sudden expansion creates a pressure wave in the working fluid. A portion of the pressure wave propagates to the ink within the ink chamber 28, and causes an ink droplet to be expelled through the nozzle 27. When the voltage pulse ceases, the bubble collapses and the flexible membrane 29 moves downward in the direction of downward arrow 33. Ink drop ejections can be generated repeatedly by controlling the voltage pulses applied to the resistor 23.
Bubble-driven-flexible-membrane-type drop ejectors have the advantage that the ink itself is not exposed to extreme heat and vaporization. Therefore, the ink can be formulated for good image-forming properties, and the working fluid can be formulated for good bubble nucleation and growth properties. However, inclusion of a flexible membrane adds manufacturing complexities and costs. In addition, repeated cycles of stretching and relaxing of the membrane can cause material fatigue, resulting in reduced device reliability and degraded performance. Furthermore, compared to conventional thermal inkjet, additional energy is required to deform the membrane for transferring the pressure wave from the working fluid to the ink, so that energy efficiency is decreased. Also, the membrane presents additional fluidic impedance to the working fluid moving toward the nozzle 27 in the direction of upward arrow 32, so that as the bubble expands, a greater amount of pressure and working fluid is directed toward working fluid channel 30. This can cause undesirable fluidic crosstalk in the working fluid passageways (working fluid channels 30 and working fluid chambers 25) of neighboring drop ejectors. In addition, for greater responsiveness of the membrane, it can be advantageous to design the membrane, working fluid and ink to form an underdamped system. However, when the flexible membrane 29 moves downward in the direction of downward arrow 33 in an underdamped system, it does not stop in the rest position shown in FIG. 1, but rather overshoots the rest position due to elastic restoring forces and the membrane 29 bulges somewhat toward the resistor 23. This tends to push additional working fluid from working fluid chamber 25 into working fluid channel 30. This wastes energy and also can cause additional undesirable fluidic crosstalk in the working fluid passageways of neighboring drop ejectors. As a result, the maximum allowed frequency of stable drop ejection can be decreased, so that the printing throughput is reduced.
Despite the previous advances in the use of working fluids to provide the drop ejection forces from heating elements to inks having poor compatibility with conventional thermal inkjet drop ejectors, improved systems and methods for ejecting drops using working fluids are still needed for reducing manufacturing complexities and costs, for improving reliability, for increasing energy efficiency, and for increasing printing throughput.